Instrument malfunctions typically manifest as abnormal indication values, such as excessively high or low readings, no change, or abnormal fluctuations. The causes of these faults fall into two categories:
- Abnormal process parameters due to unreasonable settings.
- Measurement system failures leading to inaccurate readings.
To ensure normal instrument operation, accurate fault diagnosis and analysis are essential. This requires:
- Knowledge of the measurement principles, instrument selection, and operational characteristics.
- Familiarity with the production process, material properties, and equipment/container characteristics.
- Understanding the workflow of the measurement system.
01. Fault Diagnosis Methods for Automation Instruments
Common diagnostic methods include:
- Visual Inspection: Check for physical damage to the instrument or cables, and ensure buttons/knobs on control panels function properly.
- Internal Inspection: Examine internal connectors, terminals, and pins for oxidation, corrosion, or poor contact. Verify the condition of fuses and relays.
- Voltage/Current Measurement: Use a multimeter to test voltage, current, and resistance of components.
02. Fault Analysis and Solutions for Common Instruments
1. Temperature Instruments (Thermocouples & Thermal Resistors)
- Thermocouples: Errors may arise from mismatched compensation cables or incorrect polarity.
- Thermal Resistors: Suitable for low-temperature environments. For measurement errors (e.g., max/min readings), first check the resistance or thermoelectric signal at the sensor. Then inspect for short circuits, open circuits, or poor contact in wiring.
2.Dual-Flange Differential Pressure Transmitters for Liquid Level Measurement
Measurement
Common issues post-installation include significant measurement errors or fluctuating readings. Causes:
- Capillary tube rupture causing silicone oil leakage.
- Silicone oil expansion/contraction due to ambient temperature changes.
- Medium crystallization or high viscosity.
- Process-related density variations.
Solutions:
- Stabilize capillary temperature with insulation.
- Recalibrate the transmitter zero point for sensor drift.
- Adjust transmitter range parameters for density changes
3.Control Valves
Common issues:
- Valve fails to actuate: Check air supply (blockages, pressure), DCS output, diaphragm leaks, or stem jamming.
- Slow actuation: Inspect stem/valve core deformation, diaphragm rupture, or low air pressure.
- Excessive leakage: Replace worn valve cores, adjust stem length, replace corroded springs, or clear blockages.
- Fluctuations: Optimize PID parameters, secure linkage between positioner and stem, fix air leaks, or repair signal lines.
4.Flow Meters (No Display)
Check：
- Signal/power line connections.
- Loose display module pins.
- Damaged transformers or fuses.
- Grounding issues in signal lines.
5. Pressure Instruments
- Fluctuating readings: Confirm process parameter changes or PID anomalies.
- Static readings: Check for blocked impulse lines or faulty output modules.
- Inaccurate readings: Relocate instruments away from electromagnetic interference.
6. Signal Cable Faults
Causes:
- Cable damage during installation (e.g., abrasion, insulation degradation).
- Proximity to high-temperature equipment leading to burnout.
Preventive Measures
- Avoid rough handling during cable laying.
- Route cables away from high-temperature zones. Use protective conduits or trays.
Maintenance Recommendations
- Establish and enforce regular inspection protocols.
- Conduct daily checks for leaks, cleanliness, and mechanical wear. Lubricate moving parts and address corrosion promptly.
- In corrosive/dusty environments, maintain seals, transmitters, and connectors to ensure reliability.